<?php
    function params_to_string($params) {
        if (is_array($params)) {
            ksort($params);
           return join(array_map(function($key, $value){ return $key."=".params_to_string($value); },
               array_keys($params), $params));
         } else {
             return $params;
         }
    }

     function sign($params, $private_key) {
          return md5(params_to_string($params).$private_key);
     }


#EXAMPLES===============================================================
$private_key = '40fac5841bd7d67740e61ff36cd166ad';

$params = array('begin_date' => "2017-12-16", "end_date" => "2017-12-18");


$sign = sign($params, $private_key);

$params["sign"] = $sign;

$url_for_post = 'https://realtycalendar.ru/api/v1/bookings/3bc48ca2dd73e6eb8aa42a458ad3dabb';

# send post with $params to $url_for_post